This proposal focuses on the flagellum of Salmonella typhimurium and how its external components are exported and assembled. With just two exceptions (the proteins of the periplasmic P ring and the outer membrane L ring, both of which utilize the primary Sec secretion pathway), external flagellar proteins such as the rod proteins, hook protein, and filament protein are exported by a dedicated, flagellum- specific pathway. The individual protein subunits are translocated across the plane of the cell membrane, and then travel down a central channel within the nascent structure and assemble at its distal end. Where and how do the exported proteins cross the membrane? Although the concept may seem strange at first, the most likely location for the export apparatus in a "patch" of specialized membrane at the center of the basal-body MS ring, which lies in the cytoplasmic membrane. In fact we have shown recently that two of the export apparatus components (FliP and FliR) are membrane proteins associated with the basal body and that FliR, at least, is located in the cytoplasmic face of the MS ring. The known or suspected components of the export pathway-about ten in number-are likely to fall into two or perhaps three major classes: membrane proteins that form a complex within the pore of the MS ring, peripheral membrane proteins that either stably of transiently associate with the complex and function either as chaperones or as energy sources for the transport process, and proteins that reside entirely in the cytoplasm. We hope to establish (i) whether the other membrane components of the apparatus (FlhA, FlhB, FliO, and FliQ) are physically associated with the basal body; (ii) which proteins interact with each other; (iii) whether there are proteins that are specifically required for export of a given exported protein; and (iv) to what degree the export process (as opposed to the assembly process) is an ordered one. We also hope to carry out structural studies of selected components of the apparatus and ultimately to attempt in vitro reconstitution of the apparatus. The importance of the proposed research is enhanced by the fact that the flagellar export pathway and many of the pathways by which pathogenic bacteria export virulence factors all belong to a common family, the type III secretion pathways.